Tone and density compensating device



Feb. 13, 1951 F. A. HESTER ET AL TONE AND DENSITY COMPENSATING DEVICE 3 Sheets-Sheet 1 Filed May 18, 1948 fimmaw mw M0025 UZEUPME IullluI MDOW FRANK A. HESTER JCHN W. SMWEHI IN V EN TOR.

Feb. 13, 1951 HEsTER r AL 2,541,060

TONE AND DENSITY COMPENSATING DEVICE Filed May 18, 1948 5 Sheets-Sheet 2 PHOTO CELL OUTPUT l I I I l l I LOG (PHOTO CELL SENSITIVITY X LIGHT INTENSITY) SUB-CARRIER 55 SOURCE 7 uTlLlz A'nou q MEANS FIGS.

FRANK A. HESTEIR JOHN W. SMITH INVENTOR.

ATTORNEY F. A. HESTER El AL 2,541,060

TONE AND DENSITY COMPENSATING DEVICE 3 Sheets-Sheet 3 Feb. '13, 1951 Filed May 18, 1948 BEzou MUEDOW 4(205 FRANK A. HESTER JOWN w. SMITH INVENTOR. BY Q M ATTORNEY Patented Feb. 13, 1951 um'reo STATES PATENT OFFICE Frank A. Hester, New York, and John W. Smith, Whitestone, N. Y., assignors to Faximile, Inc., New York, .N. Y., a corporation of Delaware Application May 1a, 1948, Serial No. 27,646

7 Claims. (01. 322-3) The present invention relates to electrical recorders, in particular, to means and methods of tone scale control and copy tone correction in transmitters and electrolytic recordersand similar devices.

In facsimile scanners, and the like, subject copy, or an original, consisting of graphic material, text, and half -tone material such as pictures, is scanned with a small spot of light and the resulting variation in the reflected or transmitted light, produced by variations in the density of the elemental areas making up the Original, are transformed into electrical signal variations. One normal method of using these electrical variations is to have them modulate a subcarrier signal in a balanced modulator, which subscarrier signal in turn modulates a radio transmitter or is directly transmitted by wire. Th intelligence is thus transferred to a distant point and recorded on a recording scanner by reproducing the signal variations, in either positive or negative phases, point by point or line by line in synchronism with the transmitting equipment. A true facsimile is produced when the density vari ations of the reproduction bear a definite relaconcern since it is therein common to have the "subcarrier modulated from zero output for sigtionship to the density variations on the original.

An original consisting of black and White subject matter, such as textual matter or line drawings, is very easily transmitted and reproduced since the recorded density is either minimum or maximum, depending upon whether th point being scanned is white or black. However, because the electrinature, the equipment being used must be capable of producing electrical signals which vary linearly in their relative intensity in the same manner as the variations in subject density (the logarithm of the ratio of the reflected to the incident light). In other words, the system of scanning and conversion or light signal variations into electrical signals must produce linear electrical variations for logarithmic light variations. The present invention is concerned with a method of assuring that the electrical variations 4 signal source and (2) the balance point of the cal signals are directly used to produce the reproductions, when the original is of a half-tone nals corresponding to one density, to a predetermined output for the signals corresponding to the contrasting density, Thus, in the facsimile system described herein, the system is adjusted so that a signal representative of white will not produce a subcarrier output, whereas a signal representative of black will produce a full subcarrier signal. Therefore, it is necessary to have the same input signal strength for the balanced modulator circuit for all facsimile signals of the same type, regardless of the nature of the original subject matter being used. This invention is also concerned with a method of maintaining such constancy, though the original subject matter scanned may produce varying electrical signal strengths in the photoelectric conversion system for similar shading effects.

It was stated above that it is. essential that the apparatus, for converting light variations into electrical variations should be capable of producing linear electrical signal functions representing the logarithmic light variations. It is equally essential, when certain types of originals are being scanned, and corrective measures would improve their quality, to have some means of varying the linearity of signal conversion so that densities of one type may be distorted to produce an overall reproduction that is of a better density contrast than th original. A situation of this type arises when an original having a dull white is being scanned or, of an opposite nature, an original having a dull black is being scanned. In the one case it is desirable to have the dull white changed to a pure white signal, in the other cas it is desirable to have the dull black density intensified to a pure black signal. In between the dull white and the dull black there may be variations of white or black in the original which would classify the subject matter as inferior but, if varied in a certain manner, the contrast range between black and white could be changed to improve the quality of the original in any reproduced copy. This invention is also concerned with a method of controlling the conversion of the reflected light signals into electrical signals so that such corrective measures may be made.

Thus, it is on object of the present invention to provide a method of and means for simply and semi-automatically modifying input signals of varying characteristics so that they will comply with the predetermined characteristic requireassaoeo a ihentsofthe remainderoithecircuitsystemand function in a predetermined manner.

Another object of the present invention is to provide a system for translating the density vari-.

ation in original su iect matter into electrical signals linearly related thereto and for compressing either end of the electrical signal scale.

Another object is to provide a system wherein the variations in the quality of black and white densities, between originals being scanned, will have little or no eflect on the signals transmitted or the reproduction signals received at the receiver, the reproduction being of at least standard quality in spite of deficiencies in the original.

Still a further object is to provide a system for semi-automatically expanding or contracting portions of the scale in order to compensate for deficiencies in the tone scale of the original.

A still further object is to accomplish the above named objects in a system embodying a balanced modulator and, at the same time, to provide a simple and simultaneous method and means for eflecting the desired setting and for maintaining the balance in the modulator system.

A still further object is to adjust the quality of white to conform with any predetermined white standard ,while at the same time keeping the linearity of response over the entire tonal range from white to black in conformance with a standand response, solely by V rying the photocell sensitivity.

These and other objects of the present inventionwillbeapparenttothoseskilledintheart from the detailed description of the inventions given in connection with the various figures of the drawing.

In the drawings: a

Pig. 1 shows a simplified circuit diagram of one embodiment of the present invention.

Fig. 2 shows a response curve characteristic of ttlilem. photocell scanner used in the present invenl 'ig. 3 shows a simplified drawing oi a facsimile scanner embodying the present invention.

Fig. 4 shows a simplified drawing of another embodiment of the present invention.

InI 'ig. 2 there is plotted a representative curve of photocell output voltage against the logarithm of the product or photocell sensitivity multiplied by light intensity. To establish this curve, many photocells were tested for the purpose oi determining the changes in output characteristics of a photocell, having generally a logarithmic type of output, when the input light intensity is varied or the biasing voltages on the dynodes are varied. Circuits oi the type used for obtaining logarithmic photocell outputs are fully described in the patent application of Frank A. Hester, entitled, Logarithmic Circuit" filed in the Patent Oilice March 22, 1946, and bearing Serial No. 656,216. It was found that, for any output value of the photocell circuit, one variable, say light intensity, could be varied to change the output of the cell, and the dynode biasing voltage could be varied to cancel out the effect of the change in light intensity, thus bringing -.the output voltage back" to the original value. Over the normal range of 4 and changes of slope, occur at the same output voltages, and are independent of photocell sensitivity. It was found that, in a circuit of the type shown in Fig. 1, point A on the curve oi Fig.2 being a voltage 0! approximately 0.6 volt, the path A-A' was the best operational portion of the curve for standard operating conditions. when linearity of reproduction was desired. Normally, when the electrical i al scale of any original subject matter is adjusted to cause the scale to fall between A and A, the best conversion conditions are obtained. It the photocell dynode biasing voltages are adjusted so that the cell operates along a diiierent path of operation. say from C to 0', there is a distortion or bunching effect of the white, and near white values, with respect to the black and shaded areas of the original being used. Similarly, photocell constants may be varied to place the operational path oi the cell along the B to B path producing similar results on the lower end of the scale. It can easily be seen that if an ori inal, having what might be called standard white and standard black, is used to obtain the pointsA and A, an-

other original with a white portion of a higher intensity, thus producing a hi her signal voltage,

would cause the operational portion of the curve to fall between C and C, thereby producing distortion and non-linearity. Or,'if an original is 3-3, producing distortion of the shaded and I black portions of the subject matter, if they are correspondingly dark. As stated above, it is found desirable to be able quickly and simply to adjust the photocell output so that any subject matter will produce signals along any predetermined dcsired portion of the curve. In Fig. l a system and a circuit for simultaneously making these adjustments and obtaining balance in the balansh ced' modulator, atthe desirable points, is

own.

Inli'ig. l. the reflected light or light transmitted through the copy of subject matter being scanned acts upon electron multiplier photocell I, so as to produce a signal across resistor II, which signal determines the voltage on grid ll of cathode follower i'l. Electron multiplier photo cell I has a cathode 2 and accelerator dynodes I, l, 5, t, l and 8 and finally dynode 8. Each of these dynodes, except for dynode l, is maintained at a correspondingly higher voltage by means of potentials derived from dynode power supply 39 and at a voltage determined by the relative values of bleeder resistors III, II, l2, l3, l4 and II. Final collector dynode s is maintained at the same "no-signal bias voltage as dynode t by connecting it through load resistor ii to ground It. As described in the patent application referred to above, the voltage developed across resistor 2|, due to light variations impinging upon cathode 2; will bear a logarithmic relation'to those light variations and will, therefore, function linearly. The voltage developed across pthe' resistor 2| is applied to grid it of cathode the photocell, Fig. 2 demonstrates that it is posfollower l1, which has plate 20 at a suitable positive potential and cathode it connected through resistor 54 to ground It. The signal on grid It produces a corresponding voltage across resistor It which is applied to the centerpoint of the balanced modulating transformer network made up of windings it, 51, 58, N, N and II. The input circuit of the balanced modulator is a transformer coupled to subcarrier source It, and the output is coupled to load resistor 13. The center tap of the primary oi the balanced modulator output transformer is connected to the cathode circuit oi cathode follower 23. The subcarrier voltage to be modulated is supplied by subcarrier source 55 through winding 55. Balanced modulation is achieved by means of the switch-connected rectifiers 59, 60, 6| and. The manner in which this modulator operates is set forth in Patent No. 2,313,583 issued to Hugh C. Ressler on March 9,1943. A direct current voltage, to balance the steady state or white signal voltage across resistor 54, is supplied through the center tap of windings 63 and 64 from the cathode circuit oi the balance tube 23. Balance tube 23 has cathode 24, heated by a conventional means not shown, control grid 25, connected to ground It, and plate 26 connected to a suitable positive potential source such as 22. The constants of the balanced modulator are such that it is balanced to produce no output in the output circuit when a standard white signal is being used or scanned by photocell i, or a voltage of approximately 0.6 volt is developed across resistor II, when contact arm 35 is resting on switch point 32. Variable contact 35 makescontact with fixed contacts 23, 30, 32, 34 and 31 and is in turn connected to the center tap of transformer winding 43 and 64 in the balanced modulator. The load resistor I9 is part or a potentiometer and, by means of wire leads 30, is connected to amplifier 43. The output of amplifier 36 may be connected over leads 32 and .83 to a metering device 3!. The white balance control, or dynode ,biasing voltage, for photocell l is connected to variable contact 49 which contact makes contact with fixed contacts 4|, 43, 45, 41 and 43. Contacts 4|,43, 45, 41 and 43 are at respectively higher potentials across resistors 40, 42, 44'and 43; Contact 4|, and the lower end oi resistor 40, are connected to variable contact 52 which in turn is 33 are connected as av load-circuit'across power supply '33, with 'the"lcw er end of resistor 53 being connectedtcground it. Variablecontact 43,

acting-with contacts. 41, 43, 45,141 'and.43,-permits the application to'ithe dynodes oi photomultiplier l of any predetermined bias voltage. Potentiometer 32-'5l permits a line adjustment of the dynode bias settings. The voltage developed across resistor 54 determinesthe status of operation of the balanced modulator and is representa- 6 tive measures is being used, sans switch It is set so that contact 43 is in contact with contact 45 and contact 35 is connected to contact 32. If an output signal is produced, when white is being viewed by the photocell, potentiometer 52--5l is varied to give a no-output condition, as measured bymetering circuit 3|. Having obtained a no-output signal for white, the amount of signal desired for full black is set by adjusting potentiometer 19 when the photocell is viewing black of the subject matter. Having thus established the extremes of voltage variation, values of light density in between black and white will have corresponding voltage values. Thus, it is seen that the only operating adjustments required to eirect linear conversion of light signals to electrical signals, and to adjust the electrical signals so that all white signals produce balance in the modulator, are the setting of gang switch 53, and the adjustment of white balance control 52-5l, and black level control potentiometer 13.

It was stated above, in the description 01' Fig. 2, that the preferable path of operation along Fig. 2 is from A to A. Such a path of operation will produce linear electrical signal variations with negligible bunching or distortion. It was also stated that the path of operation depended upon a number of variables, one being the dynode biasing potentials oithe photomultiplier i. The switch 50, ganged to operate variable contacts 49 and 35 so that they will make contact with a set of fixed contacts, determines the photomultiplier biasing voltage and the balance modulator voltage respectively for different conditions of operation. As described above, when contact 48-is in contact with contact 45, and 35 is in contact with contact 32, the conditions for standard operation are established. It; as described under Fig. 2, it is desirable to operate along the path C-G', or B-B'; of the sensitivity curve of the-photocell, for the purpose of eil'ecting some correction of the subject matter being scanned, then gang switch is set so that con-- tacts 49 and either 4i or 43 and contact 35 and either:3lor 34 are connected, it the subject matter is ofa very high contrast and low d.ensity'.-

The selection of either contact 34 or 31, and 4| fai tive of the electrical signal voltage developed by the photomultiplier'tube. The point of balance in the balance modulator circuit is determined by the position oi variable contact 35 with respect to ilxed contacts 28, 30, 32, 34 and 31. These fixed contacts are tapped: positions across thecathode resistor or cathode follower 23, which cathode resistor consists of resistor elements 21, 23, 3|, 33, 36 and variable resistor 33, the latter being connected to ground It.

The principal control of the fixed circuit constants is variable resistor 33, since it has a vernier control over the amount 0! balance voltage required to balance the modulator circuit. Preliminary to all transmission or operation, resistor 33 is adjusted to effect balance in the modulator when a voltage of 0.6 volt is obtained across resistor 2|, with variable contact 35 set on fixed contact 32. This setting, when once made, is only reset when some of the circuit components are changed. It is not an operational adjustment but a factory or engineering adjustment.

To operate, ii an original requiring no corrcc or 43, will depend upon the amount of correc tive measure desired. Similarly, it the subject matter is predominantly dark and of low contrast, gang switch so is adjusted so that contact 43 will make contact with contacts 43' or 41 and contact. with contacts 28 or 3B, depending upon the amount of corrective measure desired. The interaction of the dynode biasing, or white balance switch, having variable contact. 49, and the balanced modulatorpotential control, with variable contact 35, produces a proper balanced modulator potential for each bias setting. In all situations, gang switch 50 selects the predetermined path of operation and variable potentiometer 5l-52 makes the vernier adjustment to producea result of no-output voltage for f curve the system would operate while still maintaining proper balance voltage in the balance tion as set forth in the appended claims.

modulator circuit. The present invention sets forth a method and means for adjusting the photomultiplier dynode biasing potential and balanced modulator potential so that any one of a number of pre-set conditions can be repeated,

regardless of what type of original subject matter is being employed, so as to produce reproductions with continuing good fidelity. All of this is done simply by operating gang switch 50 to select the type of reproduction that is desired,

and varyingpotentiometer 5l-52 to adjust the white signal output to the balance voltage circult. Thus, by solely varying photocell sensitivity, the presentinvention achieves, simply,

results obtained in the prior art only by the use of many and varied controls.

. Fig.3 shows, in simplified form, one form of a complete facsimile scanner and transmitter utilizing the present invention. The facsimile scanner includes a copy drum 61 driven by a suitable motor 68, light from the light source II illuminating scanning spot 13, and a photoelectric device, as described in Fig. 1, contained in unit ll. The photoelectric device in unit I4 is connected by wires 15, it and I1 to unit 10, which includes the balanced modulator, power supplies and control mechanism described in Fig. 1. Subcarrier source 55 is shown and the modulated output of unit II is applied to utilization means as wherein the load resistor, amplifier and metering circuit shown in Fig. 1 are assembled.

Fig. 4 shows a modified embodiment of the present invention as described in Fig. 1. In Fig. 1 a logarithmic photocell circuit, producing linear signals across an output resistor was described.

In Fig. 4 a photocell, or other source of similar signals, produces, in signal source It, normal, unmodified signals that vary exponentially with optical density.- These signals are applied across rid 01 and cathode ll of vacuum tube ll. Plate 4 I of vacuum tube ll is connected through resistor ii to the cathode it. Resistor ll compares with resistor 2i of-Fig. 1. produced across resistor Ii are applied to the cathode follower If, in a manner similar to that shown in m. 1 and, as in rm 1, through cathode followers! to balanced modulator O3 and output amplifier :4. The output of the signal source It is controlled by control means sl, in any suitable manner. This control may be mechanically coupled to balanced modulator control so as in Fig. 1. The operation of this embodiment is" similar to that described in Fig. 1, except that the signal output of signal source 86 is exponential in nature and is fed through a logarithmic 5 amplifier consisting of vacuum tube as and resistor II. The resultant output consists of linear signal variations proportional to the density of the subject being scanned. The action of a tube of this type is also completely described in the patent application referred to above. Thus, it can be seen that the present invention can'be used with a photocell circuit producing linear signals, as in Fig. 1, or with a photocell circuit producing exponential signals, cooperatc5 above. 7

While only two embodiments of the present invention have been shown and described, many variations will be apparent to those skilled in the art within the spirit andrscope of the inven- The signals by upper and lower bends separated by an essentially linear region, means for directing light to be converted into said photoelectric device. a source of adjustable bias for setting the path of oper-. ation in. said photoelectric device, a balanced modulator circuit in which-the output signals of said photoelectric device modulate other signals,

means for varying the balance voltage of said balanced modulator, last said means being coupled to said means of adjustable bias tosimultaneously set the path of operation in said photoelectric device and to produce the proper balance voltage in said balanced modulator circuit for said path of operation. Y

2. In a signal system, the combination of, a source of input signals, means for varying the signal strength of said input signals, a signal translating circuit the input of which is connected to said source of signals and the output of which is characterized by a curve having linear and nonlinear portions, a source of higher frequency signals, a balanced modulator connected to said source of higher frequency signals and to the output of said signal translating device in which the signals from said translating device modulate the higher frequency signals, means for determining the balance point in said modulator, said means being coupled to the means for varying the signal strength of said input signals so that the position of operation of said input signals on the output curve of said signal translating device and the, desired point of balance in said balanced modulator may be simultaneously selected.

3. In a system for converting light variations into electric signals, the combinationof, a photo- 0 electric device having a response characterised by upper and lower bends separated by an essentially linear region. means for directing light to be converted into electrical signals into said photoelectric device, a source of adjustable bias for setting the path of operation in said photoelectric device, a balanced tube circuit for receiving said electric signals and means for maintaining said balance with changes in said path of operation, in which at least part of said source of adjustable bias and part of said means for maintaining said balance are mechanically coupled.

4. In a system for converting light variations into electrical signals, the combination of, a photoelectric device having a response characterized by upper 'and lower bends separated by an essentially linear region, means for directing light to be converted into electrical signals into said photoelectric device, a source of adjustable bias for setting the path of operation in said photoelectric device, a balanced tube circuit for receiving said electric signals and means for maintaining the balance in said balanced tube circuit with changes in said path of operation, a source of high frequency signals, and a balanced modulator circuit coupled to the output of said balanced tube circuit and to said source ofhigh frequency signals in which said signals from said balanced tube circuit modulate said high frequency signals.

5. lnsaefacsimile system, the combination of, a device for converting light variations into electrical signals having a response characterized by upper and lower bends separated by an essen- 15 tially linear region, means for directing light to be converted into said device, a source of adjustable bias for setting the path of operation in said device, a balanced tube circuit for receiving said electrical signals. means for maintaining the balance in said balanced tube circuit with changes in said path of operation, and a mechanical coupling between at least part of said source of adjustable bias and part of said means for maintaining said balance for the purpose of quickly carrier frequency in said balanced modulator and thereby modulating said subcarrier by means of said electrical signals, an output amplifier coupled to the output of said balanced modulator circuit, means for varying the bias of said bias sensitive device to maintain the product of said mean light value and sensitivity of said bias device substantially constant, means mechanically coupled with first said means for adjusting the balance voltage in said balanced modulator circuit so as to automatically maintain the proper balance voltage setting for the selected sensitivity setting 01 said bias device, and means for controlling the output 01- said amplifier for producing predetermined maximum and minimum output signal strengths for said electrical ii'mals.

7. In a system for converting light variations into electrical signals, the combination of, an electron multiplier photocell including a plurality of dynodes, means for directing light to be converted into said photocell, a source of adjustable bias connected to at least one of said dynodes for controlling the path of operation in said photocell, a source of signals to be modulated, a balanced modulator coupled to 'said photocell and said source of signals for modulating said signals from said source in accordance with signals from said photocell, a source of adjustable bias for varying the balance voltage of said balanced modulator, the last said bias adjustment being coupled to said first bias adjustment to simultaneously set the path of operation in said photocell and to produce predetermined balance voltages in said balanced modulator.

FRANK A. HESTER. JOHN W. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,738,315 Ranger Dec. 3, 1929 1,988,472 Artzt Jan. 22, 1935 2,083,245 Shore et a1 June 8, 1937 2,095,717 Shore et al. Oct. 12, 1937 2,136,606 Bendel Nov. 15, 1938 2,202,629 Hansell May 28, 1940 2,252,752 Bliss Aug. 19, 1941 2,280,303 Reynolds Apr. 21, 1942 

